Indeed... it's marked as beta, even being 5.0, not 5.0b1 or the usual naming scheme Mozilla uses for development tarballs... fortunately, I'm using IE9 in Windows and Firefox 4.0.1 under Linux, so... not much of a big loss about not using it, but apart from the version number... what's the big deal about version 5?

This type of development cycle works great for Chrome because of the extension framework. Extensions are somewhat limited, but are rarely broken with updates. Firefox's model is more robust for extensions and relies on a traditional PC/MAC infrastructure with longer development times. I understand that they've standardized the extension network a bit so we won't have as many breaks, but there is so much legacy code out there that these frequent updates are going to be a thorn in our sides for some time to come.

It is not beta, this is the final release. It's built from mozilla-releases repository.

It is beta, even if they picked the source up from the releases repository, it's part of their new development model (aurora, future and beta stages). At least, that's the conclusion I can make from this screenshot: